1. Technical Field of the Invention
This invention relates generally to wireless communication systems and more particularly to wireless receivers used within such wireless communication systems.
2. Description of Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver may be a prior art direct conversion or very low intermediate frequency (VLIF) receiver as shown in FIG. 1. Such a receiver includes an antenna, transmit receive switch (T/R), a bandpass filter (BPF), low noise amplifier (LNA), local oscillation generator (LO Gen), a pair of mixers, a 90° phase shift module, two low pass filters (LPF) for a direct conversion receiver or two bandpass filters (BPF) for a very low intermediate frequency receiver, two programmable gain amplifiers (PGA), and two analog-digital converters (ADC). The transmit/receive switch (T/R) couples the antenna to the bandpass filter (BPF) to receive incoming radio frequency (RF) signals. The low noise amplifier filters the inbound RF signal and provides a in-phase (I) representation of the RF signal to a 1st mixer and a quadrature (Q) representation of the RF signal to a 2nd mixer. The 1st mixer mixes the in-phase component of the RF signal with an in-phase component of a local oscillation to produce a 1st mixed signal. The 2nd mixer mixes the quadrature component of the RF signal with a quadrature component of the local oscillator to produce a 2nd mixed signal.
For direct conversion receivers, low pass filters (LPF) filter the 1st and 2nd mixed signals. For a very low intermediate frequency receiver, bandpass filters (BPF) filter the 1st and 2nd signals. The programmable gain amplifiers (PGA) amplify the filtered signals and provide them to the analog-to-digital converters (ADC). The analog-to-digital converters convert the analog in-phase component into a digital in-phase component at baseband or low intermediate frequency and convert the analog quadrature component at baseband or low IF into a digital quadrature signal.
The direct conversion, or VLIF, receiver of FIG. 1 suffers from DC offset that is caused by self-mixing of the local oscillation in the RF inputs of the 1st and 2nd mixers and the DC offset generated by the mixer and filter themselves. Such a DC offset produces energy at DC, which for direct conversion and very low IF receivers are in the signal band of interest. If the DC offset is too large with respect to the incoming signal, it dominates the received signal and saturates the following stages (e.g., low pass or bandpass filters, programmable gain amplifier, analog-to-digital converter) or may corrupt the received signal due to degraded Signal-Noise-Ratio (SNR).
One method to reduce DC offset is illustrated in FIG. 2. In this embodiment, the direct conversion, or very low IF, receiver includes an analog DC offset cancellation circuit. As shown, the DC offset cancellation circuit is feed back and subtracted from the incoming mixed signals. To determine the value for the DC offset cancellation circuit, the DC offset is measured during idle times prior to receiving an incoming RF signal. Accordingly, when no RF input signal is being received, and a DC value is present at the output of the programmable gain amplifiers, it is assumed to be created by the self-mixing of the local oscillation within the 1st and 2nd mixers, impedance mismatch and circuitry DC offset. As such, by measuring this value and then subsequently subtracting it when valid incoming RF signals are being received, the net effect of the DC offset is reduced. However, during the DC offset measuring time, the antenna is switched off to isolate the impact of external interference signals that may induce extra DC offset due to the limited linearities of the components of the receiver. As such, residual DC offset may be present, which, for some sensitive modulation schemes such as EDGE (3π/8 8PSK), the residual DC offset is still problematic.
FIGS. 3 and 4 illustrate another method for reducing DC offset in direct conversion, or very low IF, receivers. In this approach, the digital signal processor (DSP) buffers an entire burst of a received RF signal as shown in FIG. 4. From the buffered received signal, the DSP calculates the DC offset by averaging the entire received signal. If no DC offset is present, the average value for the burst of data should be approximately at AC ground (i.e., 0 volts AC). However, if a DC offset exists, the average value will not be at AC ground as illustrated in FIG. 4. Knowing the DC offset, the DSP may adjust the subsequent received bursts of data to accommodate for the DC offset. While this technique reduces DC offset, it also removes the DC component of the incoming RF signal.
Therefore, a need exists for a DC offset cancellation technique that reduces residual DC offsets and avoids removal of the DC component of the incoming signal.